1. Field of the Invention
The present invention relates to the field of continuous wave ultrasonic receivers.
2. Prior Art
In terms of prior art, there are only a few methods used today to implement the analog continuous wave (CW) beamformer for ultrasonic receivers. These include the following:
U.S. Pat. No. 5,555,534 discloses a technique known as the Phase Rotator. In this technique, the outputs of a quadrature downconverter (two mixers driven in quadrature) are combined by a Phase Rotator as shown in FIG. 1.
The Phase Rotator works on the principle of a weighted summing of the I and Q output signals from the mixers in such a way as to rotate their phases while maintaining quadrature (see FIGS. 2 and 3). The 534 patent gives a table of the required I and Q weighting factors from summation (Table 1).
TABLE 1Weighting Factors for Phase RotatorPhi WordI+ OutputQ+ OutputI− OutputQ− Output000+I7 − Q3+Q7 + I3−I7 + Q3−Q7 − I3001+I7 + Q3+Q7 − I3−I7 − Q3−Q7 + I3010+Q7 + I3−I7 + Q3−Q7 − I3+I7 − Q3011+Q7 − I3−I7 − Q3−Q7 + I3+I7 + Q3100−I7 + Q3−Q7 − I3+I7 − Q3+Q7 + I3101−I7 − Q3−Q7 + I3+I7 + Q3+Q7 − I3110−Q7 − I3+I7 − Q3+Q7 + I3−I7 + Q3111−Q7 + I3+I7 + Q3+Q7 − I3−I7 − Q3
As used in Table 1:
+I7 represents the input lead originating from the sqrt(2)/2 weighted output of I.
+I3 represents the input lead origination from the (1−sqrt(2)/2) weighted output of I.
−I7 represents the input lead origination from the −sqrt(2)/2 weighted output of I.
−I3 represents the input lead origination from the −(1−sqrt(2)/2) weighted output of I. +Q7 represents the input lead originating from the sqrt(2)/2 weighted output of Q.
+Q3 represents the input lead originating from the (1−sqrt(2)/2) weighted output of Q.
−Q7 represents the input lead origination from the −sqrt(2)/2 weighted output of Q.
−Q3 represents the input lead originating from the −(1−sqrt(2)/2) weighted output of Q.
To further understand the Phase Rotator method we can look at an example. To generate the I/Q vector pair (a/b) in FIG. 3, (a) is equal to +0.383Q and +0.924I and (b) is equal to +0.383I and −0.924Q. Therefore in order to make this a/b vector pair, one needs to have two different positive amplitudes of I and one positive and one negative amplitude of Q.
Access to each of the +/−Q (or +/−I) vectors are needed at the same time, which requires four weighting functions (represented diagrammatically in FIG. 2 as variable gain amplifiers).
Using a Phase Rotator, it is difficult to do the switching required at the IF due to 1/f noise concerns for low Doppler offset frequencies.
Another approach is disclosed in U.S. Pat. No. 4,140,022 and illustrated in FIG. 6. In some present equipment, the variations in compensating delays are achieved by changing taps on delay lines as shown in FIG. 4. The taps cannot be more than a small fraction of the period of a carrier wave apart if the cycles of the carrier wave are to arrive at the summing point nearly in phase. This requires the number of taps in this approach to be large. At the carrier frequencies employed, only a relatively expensive electrical delay line can be used because of bandwidth considerations, and the provision of a large number of taps on this type of line is a significant portion of the cost of the entire instrument.
Of equal significance is the fact that, unless expensive tap changing switches are used, the switching transients cause a significant amount of noise in the signals arriving at the summing point and therefore in the image produced from them.
The problem of cost and transient noise just referred to increase in severity when the carrier frequency is increased to obtain better definition, the aperture of the array is increased to obtain better focusing, or the minimum range is decreased so as to permit the examination of infants.
FIG. 5 shows a prior art approach in which phase shifting means are used in combination with coarsely tapped delay lines to achieve the phase coherence required for focusing. In this case, phase shifters are inserted between the transducers and their respective delay lines.
In the preferred embodiment of U.S. Pat. No. 4,140,022, delay line taps are not relied on for producing phase coherence. The phase changing is effected by respectively heterodyning the carrier waves from each transducer with different phases of an oscillator output that are selected so as to focus the array at one point. The intermediate frequency waves are then applied to coarsely spaced taps on a delay line system that have delays nearest to that required for precisely focusing the transducer. By selection of the frequency of the local oscillator, one of the intermediate frequency sidebands of the mixer outputs can be low enough in frequency to permit the use of surface acoustic delay lines that cost much less than an electrical delay line. The use of more widely spaced taps in this invention simplifies design problems because there are fewer taps to cause troublesome reflections. In essence, the heterodyning is used for a fine phase adjust while the coarse taps on the master delay line are used for coarse phase adjust. With this method it is possible to use delay means having fewer inputs or taps than would be required to produce the required phase coherence if the delay means alone were used.
U.S. Pat. No. 6,648,826 describes a CW beamformer in an ASIC. In this patent the beamformer may be used for either CW ultrasound receive or transmit beamforming. However the patent does not discuss the details of the actual circuit operation.